The present invention relates to a method and device of amorphous diamond like carbon thin multi-layer doping growth, and in particular, it concerns a process of forming a variable structure with mix control (sp3/sp2 ratio) to obtain a large spectrum energy gap for multifunction photovoltaic and thermal solar cells and other applications.
In recent years, amorphous Diamond Like Carbon (a:DLC, or a-C:H) films have been developed. a:DLC films are easier to deposit on large substrate areas (such as plastic), at low temperature, as compared to CVD diamond-like carbon (DLC) films. The unique properties of a:DLC films are similar to those of single crystal bulk diamonds, including: high hardness, low friction coefficient, chemical inertness, infrared transparency, high electrical resistivity, and smoothness; make them suitable to many applications.
As conventional energy sources may be depleted in the future, meeting the increasing world energy demand will certainly involve increased use of solar energy. Therefore, solar cell research has been accelerated in recent years. Although silicon and compound-semiconductor-based solar cells have dominated the market for the last few decades, low-cost, stable and highly efficient solar cells are yet to be commercially realized due to high material and production costs. However, with the emergence of carbon semiconductor materials, the situation is expected to change. Carbon is readily available in nature. Carbon exhibits outstanding properties such as: chemical inertness; high hardness; high thermal conductivity; high dielectric strength, and infrared (IR) optical transparency. Carbon atoms in a:DLC films may have three different atomic coordinates: The sp3 (tetrahedral or aliphatic), or “sp3” hereinbelow and in the claims which follow, is the typical type of bond for diamond, sp2 (trigonal or aromatic) or “sp2” hereinbelow and in the claims which follow, typical for graphite, and sp1 (linear or acetylenic) or “sp1” hereinbelow, typical for amorphous carbon.
The interesting and unique feature of carbon is that properties such as those noted above can be tuned over an unusual wide range from that of conductor graphite (˜0.0 eV) to that of insulating diamond (˜5.5 eV) by varying the ratio of sp3 and sp2 hybridized bonds. Hence carbon has attracted the attention of the researchers for its application in solar or photovoltaic (PV) cells. Carbon-based heterostructures such as metal insulator semiconductor (MIS) diodes, Scottky diodes, metal insulator semiconductor field effect transistor, heterojunction diodes, and photovoltaic cells on silicon have already been reported; thereby indicating the potential of carbon materials in electro-optic devices. However, relatively few publications are available on the PV properties of C/Si hetero-structures. It is worth noting that virtually all the researchers working with carbonaceous photovoltaic cells reported the overall photoresponse of their cells. But the separation of respective contributions of carbon and silicon in a C/Si PV cell is yet to be realized. At present, there is a need to advance work on the spectral photoresponse characteristics of C/Si heterostructures and to visualize the contribution of C and Si separately to understand the nature and improve the characteristics for the practical implementation of a:DLC-based PV cells.
Previous studies, such as Aisenberg, S., Kimock, F. M., Mater. Sci. Forum, 52-53, (1988), 1, incorporated herein by reference, show that the energy of the carbon species generated by various preparation methods is different and plays an important role in controlling the sp3/sp2 ratio. Also It is recognized that the population concentrations of sp3 and sp2 bonds are also dependent on different kinds of precursor materials, which dictate the sp3/sp2 ratio of a:DLC structures films. Hence the properties of thin carbon films depend on the method of deposition, deposition parameters and precursor materials used.
There are a number of prior art dealing with the use of amorphous carbon and/or diamond-like carbon layered structures and their fabrication, among them: U.S. Pat. Nos. 6,078,133 (Menu et al.); 7,214,600 (Won et al.); 5,206,534 (Birkle et al.); and 5,366,556 (Prince et al.), incorporated herein by reference. In addition US Patent publications US2007/0042667 and US2006/0078677, herein incorporated by reference, also deal with diamond-like carbon structures and devices. While these prior art touch on many aspects concerning overall fabrication and some precursor details, detailed control of the sp3 and sp2 bonding levels (and, as a result, the sp3/sp2 ratio) is not disclosed.
An additional parameter making carbon films suitable candidates for PV cells is activation energy. Studies, such as J. Robertson, Adv. Phys., 35 (1986), 317 and C. Benndorf, M Grischke, A. Brauer and F. Thieme, Surf. Coat. Technol., 36 (1988), 171, both incorporated herein by reference, for a:DLC films (such as those used in PV cells) show that a deposited doped a:DLC film increases activation energy as compared with undoped film. Study of activation energy reveals that the Fermi level of the carbon film moves from the valence band edge to near the conduction band edge through the mid-gap. The Tauc gap, and conductivity are also influenced with film doping.
Ingram et al. in U.S. Pat. No. 5,562,781, herein incorporated by reference, describes a PV cell comprising a plurality film layers, at least one of the layers being a semiconductor film of amorphous hydrogenated carbon. PIN junctions are formed of films; all made of amorphous, hydrogenated carbon and vary only by dopant levels without each PIN junction. There are variations in band gap from one PIN junction to the next in order that the photovoltaic effect in each PIN junction will be caused by a different portion of the spectrum of light. Ingram describes an arc-discharge deposition technique, which apparently spans various stages in fabrication and he refers to conventional doping to control material properties.
There is therefore a need to for better and/or novel control of the sp3/sp2 ratio and utilizing doping in a:DLC films to fabricate improved photovoltaic cells.